1. Field of the Invention
The present invention relates to a zoom lens which is a lens system comprising a plurality of lens groups and in which part of the lens groups other than the front lens group which are disposed near an object is moved for focusing. In particular, the present invention pertains to a zoom lens suitable for use in a photographic system, such as a single-lens reflex camera having an automatic focusing detection mechanism, still video camera and a video camera.
2. Description of the Related Art
Development trends concerning zoom lenses have been directed toward a high-power, compact and inexpensive zoom lens which can be used for almost all applications and toward a high-performance zoom lens which has an aperture ratio and an image quality as high as those of a single lens. Recently, there has also been a demand for a lens technology which is suitable for use in an auto focusing camera. In the case, for example, of a telephoto type zoom lens whose focal length (F) ranges from 80 mm to 200 mm and which has an F-number (FNO) of 2.8, the front lens group (the first lens group) used for focusing has a diameter of 70 mm or above, and this makes the overall zoom lens very heavy, resulting in slow focusing and deterioration of the control of the zoom lens.
An internal focusing type zoom lens is suitable for use in an automatic focusing camera, and various types thereof have been proposed recently. However, most of them have an F-number ranging from 4 to 5.6, because internal focusing type zoom lenses having a large aperture ratio suffer from aberration caused by focusing.
Japanese Patent Laid-Open Nos. 109952/1977, 57815/1980 and 117119/1980 and Japanese Patent Publication No. 53696/1986 have proposed an internal focusing type zoom lens in which a lens group disposed on the object side of a variable-power lens group is divided into a plurality of lens groups and in which part of the plurality of lens groups other than the front lens group is moved for focusing. Such an internal focusing type zoom lens in which part of the lens groups disposed on the object side of the variable-power lens group is moved for focusing is characterized in that no shifting of focus occurs even if the zoom lens is focused at any zooming position and in that it is focused on a certain object at any zooming position by moving the focusing lens group thereof by a fixed distance.
Furthermore, the internal focusing type zoom lens has a focusing lens group which is lighter in weight than the first lens group in a zoom lens which is moved altogether for focusing, and therefore requires lower torque to drive the focusing lens group when the zoom lens is, for example, applied to a camera having an automatic focusing detection mechanism. This enables speedy focusing to be achieved.
In general, a telephoto type zoom lens suffers from variations in aberration caused by focusing. In particular, the internal focusing type zoom lens suffers from a large degree of variations in the aberration caused by focusing, although it has the above-described advantages. This makes it very difficult for a high optical performance of the zoom lens to be maintained over the entire object distance.